Rework how guest nicks work.

- Use the config setting as a string template instead of as a prefix.
- Allow users of IRCds that have UIDs to use that as the guest nick.
- Fall back to a UID before killing if a guest nick can not be found.

include/modules/pseudoclients/nickserv.h

@@ -19,4 +19,5 @@ public:
 	virtual void Validate(User *u) = 0;
 	virtual void Collide(User *u, NickAlias *na) = 0;
 	virtual void Release(NickAlias *na) = 0;
+	virtual bool IsGuestNick(const Anope::string &nick) const = 0;
 };